The present invention relates to a hydrocarbon sensor for detecting hydrocarbon and measuring the concentration of the hydrocarbon in an atmosphere in the temperature range of ordinary temperature to high temperature (800xc2x0 C.).
There are, in the art, known hydrocarbon sensors which are capable of detecting hydrocarbon in living environments, and in exhaust discharged from automobile engines, stoves and catalytic combustion apparatuses, which include catalytic converters or reformers, and are capable of being used for lean-burn control of combustion engines.
A method for measuring or detecting hydrocarbon has used a thin electrolytic substrate formed of a proton conductor as a solid electrolyte, i.e., a detecting medium. In this method, platinum electrodes are formed on both sides of the substrate so as to be opposite to each other, thereby forming a hydrocarbon sensor. Hydrocarbon in an atmosphere under measurement is dissociated into protons at the anode and the protons pass through the electrolytic substrate and reach the cathode, then causing the current or voltage to generate across both the electrodes which is detected.
An oxide-based proton conductor that can be heated and used at high temperatures over room temperature has been used for the electrolytic substrate to use such hydrocarbon sensors for combustion engines or other high-temperature apparatuses. In recent years, a calcium zirconate-based oxide having a composition of CaZr0.9In0.1O3xe2x88x92xcex1 has been developed as the oxide-based proton conductor, and attempts have been made to apply the oxide to hydrocarbon sensors. For example, as a hydrocarbon sensor comprising a solid electrolyte formed of the calcium zirconium-based oxide, an electromotive-force-type sensor is disclosed by Hibino, Tanaki and Iwahara in the Proceedings of the 61st Conference (1994) of Electro-chemical Society of Japan, p99, in which Pd and Au electrodes are used on the main faces of the solid electrolyte.
Furthermore, a limiting-current-type sensor provided with a diffusion-determining portion made of porous alumina is disclosed by Inaba, Takahashi, Saji, Shiooka in the Proceedings of the 1995 Autumnal Conference (1995) of Japan Association of Chemical Sensors, p145.
Generally, in the limiting-current-type sensor, an anode made of platinum is disposed on one face of a thin electrolytic substrate having a proton conductivity, and a cathode made of platinum is disposed on the other face, so that the anode and the cathode are opposite to each other in contact with the substrate. The anode is provided with a diffusion-determining portion to transfer hydrocarbon molecules by diffusion from an atmosphere to the anode. The amount of the hydrocarbon to be diffused and moved to the anode surface is proportional to the partial pressure in the atmosphere to be measured. When the sensor is disposed in an atmosphere under measurement, and a constant voltage is applied across both electrodes, the hydrocarbon transferred by diffusion from the atmosphere to the anode is dissociated on the anode, whereby hydrogen ions, i.e., protons, are discharged into the electrolyte. The sensor can detect the amount of protons passing through the electrolytic substrate as a current flowing across the electrodes. The sensor uses the principle that the measured proton current is approximately proportional to the concentration of the hydrocarbon in the atmosphere.
However, the solid electrolyte made of the above-mentioned calcium-zirconium-based oxide has a low proton conductivity of about 5xc3x9710xe2x88x924 S/cm at 600xc2x0 C. In order to raise the sensitivity of sensors, the operation temperature of the hydrocarbon sensor must be set at a high temperature of about 700xc2x0 C. in the case of an EMF type-hydrocarbon sensor, or the solid electrolyte must be made thinner in a thin film in the case of a current-detecting-type hydrocarbon sensor. Otherwise, it is difficult to use the sensor because of low detection sensitivity. For these reasons, solid-electrolytic materials having higher proton conductivity have been demanded.
Furthermore, problems are also caused with respect to the detection mechanism and structure of the sensor. The EMF-type hydrocarbon sensor used conventionally cannot accurately detect hydrocarbon contents in an atmosphere in which no oxygen is present or the concentration of oxygen changes significantly, since the sensor utilizes the oxygen catalytic function of the electrode. The conventional limiting-current-type sensors comprising a diffusion-determining portion made of porous alumina have difficulty in setting the electrolytic voltage for electrolyzing hydrocarbon.
The inventors of the present invention have propose d a limiting-current-type (or constant potential electrolytic type) hydrocarbon sensor formed of a barium-cerium-based oxide having high proton conductivity in Japanese Patent Publication Kokai No. 10-300718. This sensor satisfactorily responds to hydrocarbon. Further, when no oxygen is present, the sensor can nearly linearly detect hydrocarbon in the range from the order of several ppm to several percents.
However, in the case where the concentration of hydrocarbon is very low (for example, 10 ppm or less) with no oxygen present in an atmosphere, it was found that when oxygen promptly enters from the outside, the detection output of the sensor increases. This is because the barium-cerium-based oxide has a characteristic of conducting oxide ions, whereby oxygen is taken into the electrolyte at the cathode, penetrating the electrolyte to the anode. To solve this problem, the inventors have developed a sensor wherein the cathode thereof is mainly made of metal Al to prevent the entry of oxygen at the cathode in Japanese Patent Publication No. 11-337518. The Al-containing metal cathode has a significant effect on reduction of the sensor output due to oxygen even when high levels of oxygen enter in the atmosphere.
However, in an application wherein this sensor is used to detect deterioration in the performance of a catalyst used to clean exhaust from automobile engines, when the catalyst deteriorates, the exhaust includes a high concentration of hydrocarbon (HC) and a considerably large amount of oxygen (about 2.5%) mixed with the hydrocarbon. If a sensor comprising a cathode made of Al and an anode made of Au is used for this kind of application, when the concentration of the oxygen in the exhaust becomes high (0.7 to 2.7% of O2 in this example), the HC detection output of the sensor lowers as shown in FIG. 11, even though the exhaust includes a relatively high concentration of hydrocarbon (HC: 500 to 2000 ppmC), thereby causing another problem.
An object of the present invention is to provide a hydrocarbon sensor using a mixed ion conductive electrolyte which is capable of accurately measuring the hydrocarbon concentration even in an atmosphere containing high levels of oxygen, by preventing the change and reduction in HC detection current output at the time when such a high concentration of oxygen is mixed with the hydrocarbon atmosphere.
Another object of the present invention is to provide a hydrocarbon sensor comprising a cathode having a low electrode resistance.
The hydrocarbon sensor of the present invention comprises a solid electrolyte formed of a mixed ion conductor, an anode formed on one of surfaces of the electrolyte and a cathode formed on the other surface of the electrolyte, both opposed to each other, wherein the cathode is formed of an alloyed layer containing Al and a transition metal to be active to the hydrogen proton.
The transition metal is one or more elements selected from Groups 3 to 12 in the Periodic Table (IUPAC Inorganic Chemicals Nomenclatures (1989)). The transition metal may be preferably selected from active metals of Pt, Au, Ag, Cu, Pd, Mn, Fe, Ni, Co, and the like. Particularly, Au may be used as the transition metal, to provide an Alxe2x80x94Au alloy for an alloyed layer of the cathode.
For the cathode, a sintered layer, or a sintered film, may be used as an alloyed layer, which may be a solid phase or liquid phase containing Al and the transition metal. The alloyed layer of the cathode may include one or more of Al-transition metal intermediate phases, and, preferably, a metal Al phase together with the intermediate phases.
Especially, as the transition metal, Au may be used since the intermediate phases of Au and Al exhibit a property that dissociates protons into hydrogen and a property that prevents oxygen from converting into oxide ions, and also exhibit a considerably high electric conductivity required for the cathode.
The hydrocarbon sensor of the present invention does not detect current resulting from the conduction of oxygen through the electrolyte. The presence of the Al component in the cathode alloyed layer shields oxygen from the atmosphere from entering into the electrolyte by preventing the oxygen from being ionized at the cathode.
For the use of Au, even if a higher concentration of oxygen is present together with hydrocarbon in the atmosphere to be measured, the Au in the alloyed layer prevents oxidation of the cathode, and effectively prevents an increase in cathode resistance due to the oxidation of the electrode. The similar effects are provided in the transition metals by Cu, Ag, Ni, Co, Pt, Pd and other noble metals.
Furthermore, the Au component does not hinder the hydrogenation of protons, thereby preventing reduction of the proton detection current. In particular, the Au component in the cathode accelerates the association reaction of protons from the solid electrolyte at the cathode, thereby accelerating the discharge of hydrogen at the cathode. As a result, the hydrocarbon sensor of the present invention can have high hydrocarbon detection performance even when a high concentration of oxygen becomes promptly included in the atmosphere, for example, in automobile exhausts, under measurement.
For producing a hydrocarbon sensor comprising a cathode formed of this kind of alloyed layer, first paste mainly containing Au is applied to an electrolytic substrate to form a film, and the film is fired to form an Au film. Next, another paste mainly containing Al is applied to the Au film on the substrate and fired to form an alloyed layer which can be used as the cathode. By sintering, the alloyed layer comprises a first layer containing an Alxe2x80x94Au intermediate phase which makes contact with the substrate and a second layer including a metal Al phase which covers the first layer.
Another method of producing the hydrocarbon sensor may be used in which paste mainly containing Au and Al is applied to an electrolytic substrate to form a film, which is then fired to form an alloyed layer including Au and Al. The paste may be prepared of a mixture of powders of metals Au and Al. The alloyed layer is used as the cathode. This method may provide an alloyed layer including one or more of Alxe2x80x94Au intermediate phases, or an alloyed layer including Alxe2x80x94Au intermediate phases dispersed in a metal Al phase.